NEW YORK - More than half of US drivers killed in car crashes had alcohol or drugs in their system, according to the US National Highway Traffic Safety Administration.Using NHTSA data on road deaths in 14 states, researchers found that people who drove at night were the most likely to have alcohol or drugs show up on a toxicology screen after the accident.Their results appeared in the journal 'Addiction'. WEEKEND ABUSERSJoanne Brady of Columbia University and her colleagues wrote: "More than half of fatally injured drivers had taken drugs or drunk alcohol and about 20% had been using (two or more) drugs."Out of 20 150 fatally injured drivers from 2005 to 2009, 57% tested positive for at least one drug, including one in five who had multiple drugs in their system.Alcohol was the most common drug, followed by marijuana and stimulants, including amphetamines.Men (60%) were more likely to have drugs in their systmen compared to less than half of women. Drivers recorded to have had an accident at night or over a weekend were more likely to test positive than those driving during the week.Records couldn't show whether drivers had enough of a certain drug in their system to feel or act impaired or if prescription drugs were used incorrectly, experts said.Robert Voas, a student at the Pacific Institute for Research and Evaluation in Calverton, Maryland, said: "There's already is quite a bit of research that's probably going to continue to try to identify in drugs the point at which they are impairing."With alcohol, the amount of alcohol is more or less directly related to the level of behaviour impairment. The relationship of a drug in the body to the behaviour of the driver is less direct and clear."